The present invention relates to a coated cubic boron nitride sintered body in which a sintered body consisting mainly of cubic boron nitride (hereinafter referred to as cBN) is coated with a hard layer, particularly a coated cBN sintered body that is most suitable for a cutting tool having excellent resistance to wear and heat in the high-speed cutting of steel.
The hardest material next to diamond, cBN has been used in a cBN sintered body in which cBN is sintered at superhigh pressures together with a 10 to 60 vol. % binder, such as TiN, TiC, Al, or Co. These cBN sintered bodies have been available in the market mainly for use in tools for the cutting of hardened steel and cast iron.
There is another type of cBN sintered body, which is produced without using a binder. The sintered body is produced by reaction-sintering hexagonal boron nitride (hBN) with the assistance of a catalyst such as magnesium boronitride. The sintered body has a thermal conductivity as high as 600 to 700 W/mxc2x7K, enabling its use as a heatsink material and a TAB bonding tool. The sintered body, however, has some amount of residual catalyst. When heated, the sintered body tends to form ninute cracks because its thermal expansion differs from that of the residual catalyst. As a result, its maximum allowable temperature is as low as about 700xc2x0 C. In addition, because the cBN has a crystal-grain diameter as large as about 10 xcexcm, the cBN sintered body is insufficient in strength, despite its high thermal conductivity. This low mechanical strength has precluded its use as a cutting tool.
On the other hand, cBN can also be synthesized by treating low-pressure BN, such as hBN, at superhigh pressures and high temperatures without using a catalyst. This process is called direct conversion. It is known that concurrent sintering with this hBN/cBN conversion can produce a cBN sintered body containing no binder. For example, the published Japanese patent applications Tokukaishou 47-34099 and Tokukaihei 3-159964 have disclosed a method of producing a cBN sintered body by converting hBN into cBN at superhigh pressures and high temperatures. The other applications Tokukoushou 63-394 and Tokukaihei 8-47801 have disclosed another method of producing a cBN sintered body by using pyrolytic boron nitride (pBN) as the material, proposing the use of the sintered body for the high-speed cutting of gray cast iron that has relatively good machinability.
However, recent years have seen an advancement in the trend toward the high-speed cutting of steel in order to improve machining efficiency and toward high-speed cutting under dry conditions in order to reduce the adverse effects on the environment. Under such circumstances, conventional tools, such as a tool made of a coated cemented carbide, have been disadvantageous because they cause problems, such as (a) a reduced tool-life due to wear resulting from the increase in the temperature and load of the cutting edge while the tool is cutting, (b) plastic deformation of the base material, (c) crack formation due to heat shock, and (d) damage resulting from insufficient strength at high temperatures.
In order to solve these problems, another application, Tokukaishou 59-8679, has proposed a method of coating a cBN sintered body having high hardness at high temperatures with a layer of Al2O3 or a composite layer of Al2O3 and any of TiC, TiN, and TiB. Yet another application, Tokukaishou 61-183187, has disclosed a method of coating the cBN sintered body stated in Tokukaishou 59-8679 with TiN, TiC, or TiCN by the physical vapor deposition (PVD) method to improve wear resistance in the cutting of cast iron and steel.
However, these coated sintered bodies are produced by using as the base material a cBN sintered body having insufficient heat resistance and strength at high temperatures. Consequently, when they are used for the high-speed cutting of steel, the coating spalls away due to the temperature rise during the cutting, resulting in the accelerated progression of wear. In addition, the base material of these coated sintered bodies has low thermal conductivity and a large coefficient of thermal expansion. Therefore, when they are used particularly for intermittent high-speed cutting, thermal cracks are formed in the base material due to intense heat shock, without showing notable improvement in resistance to wear and chipping.
A cBN sintered body containing no binder shows excellent resistance to heat and wear when used for the high-speed cutting of cast iron. However, when used for the cutting of steel, the sintered body shows accelerated progression in the wear of the cutting edge due to the reaction between the cBN and steel, significantly reducing the tool life.
Considering the above-described circumstances, an object of the present invention is to offer a coated cBN sintered body that is most suitable for a cutting tool having excellent resistance to wear and heat particularly in the high-speed cutting of steel.
Another object of the present invention is to offer a coated cBN sintered body for long-life cutting tools by firmly bonding a hard coating having excellent resistance to wear and heat to the surface of the base material made of a cBN sintered body having excellent crack resistance at high temperatures.
The present inventors studied the heat resistance and heat-shock resistance required for a cBN sintered body to be used as a base material, and found that when conventional cBN sintered bodies are used particularly for the cutting of low-hardness steel, the reaction between the cBN particles and the ferrous material accelerates the wear, causing the development of flank wear and face wear and the subsequent reduction in tool life due to wear or the development of chipping. The present inventors also found that high-speed cutting raises the tool temperature, accelerates the softening and deterioration of the binder, reduces the resistance to heat and heat shock of the sintered body, and thereby significantly shortens the tool life. Based on the above findings, the present invention achieves the above-described objects by providing a hard coating capable of improving wear resistance on at least part of the base material made of a sintered body comprising almost solely cBN and by specifying the thickness of the coating
More specifically, the coated cBN sintered body of the present invention comprises (a) a base material made of a sintered body comprising at least 99.5 vol % cBN and (b) a hard coating 0.1 to 10 xcexcm in thickness formed on the surface of the base material by the PVD method. In addition, the bonding quality of the hard coating can be improved by providing a specific intermediate layer between the base material and the hard coating. The constituents of the present invention are explained in detail below.
The present invention features a base material containing at least 99.5 vol. % cBN. If less than 99.5 vol. %, the strength at high temperatures decreases due to the deterioration of the substances coexisting with the cBN, such as impurities, because they usually have low heat resistance in comparison with cBN. When the content of cBN is at least 99.5 vol. %, the wear resistance of the tool can be significantly improved because the reduction in the hardness of the base material is small even at high temperatures, which enables the hard coating to maintain its high hardness. It is particularly desirable that the content of cBN be at least 99.9 vol. % to secure the intrisincally excellent heat resistance of cBN.
It is desirable that a base material comprise cBN having an average crystal-grain diameter of at most 1 xcexcm. Such a small crystal-grain diameter of the cBN enables further improvement of the strength of the sintered body. If the average crystal-grain diameter exceeds 1 xcexcm, the boundary areas of the cBN grains decrease, causing an acceleration of the propagation of cracks, and thus decreasing the strength.
It is desirable that the ratio I(220)/I(111) be at least 0.05 in the X-ray diffraction lines in an arbitrary direction of a base material, where I(220) denotes the intensity of the X-ray diffraction at the (220) plane of the cBN, and I(111) denotes the intensity of the X-ray diffraction at the (111) plane of the cBN. This specification suppresses the chipping due to cleavage, enabling the tool to maintain its strength as a cutting tool.
It is desirable that a base material have a thermal conductivity of 250 to 1,000 W/mxc2x7K and a coefficient of thermal expansion of 3.0 to 4.0xc3x9710xe2x88x926/K in a temperature range of 20 to 600xc2x0 C. Such a high thermal conductivity and a small coefficient of thermal expansion can suppress the formation of cracks due to heat shock.
It is desirable that the above-described base material have a transverse rupture strength of at least 800 MPa when measured by the three-point bending method in a temperature range of 20 to 1,000xc2x0 C. and have a Vickers hardness of at least 40,000 MPa at room temperature. This specification can improve the mechanical strength of the base material at high temperatures and decrease chipping during intermittent high-speed cutting under the condition of the increased cutting-edge temperature.
The above-described base material can be produced by directly converting low-crystalline or pulverized high-purity low-pressure BN at high pressures and high temperatures. Low-pressure BN is a boron nitride that is thermodynamically stable at normal pressure. The types of low-pressure BN include hexagonal BN (hBN), rhombohedral BN (rBN), turbostratic BN (tBN), and amorphous BN (aBN). If pBN, which has intrinsically high orientation, is used as the material, the obtained cBN sintered body tends to orient in the (111) direction. This high orientation may cause laminar cracks, spalling, or other problems when the cBN is used for a cutting tool. When hBN, which has a large crystal-grain diameter, is used as the material, the rate of conversion into cBN decreases. Consequently, the production of highly purified cBN requires increasingly rigorous conditions such as higher pressures and higher temperatures, which makes it difficult to control the crystal-grain diameter. On the other hand, when low-crystalline or pulverized high-purity low-pressure BN is used as the starting material, a cBN sintered body having low orientation and a small crystal-grain diameter can be obtained under relatively mild temperature and pressure conditions. In order to reduce the average crystal-grain diameter of cBN, it is desirable that the sintering temperature be lower than 2,200xc2x0 C., more desirably between 1,800 and 2,000xc2x0 C. or so.
Low-crystalline high-purity low-pressure BN can be obtained by reducing a compound that includes boron and oxygen and using a compound that includes carbon and nitrogen. It is desirable that the direct conversion from low-pressure BN into cBN be carried out after heating the low-pressure BN at a temperature higher than the boiling point of the compound that includes boron and oxygen in a non-oxidative atmosphere.
It is desirable that a hard coating be composed of at least one layer of a compound selected from the group consisting of TiN, TiC, TiCN, TiAlN, and Al2O3, which have excellent wear resistance in the cutting of steel and cast iron. Of course, a plurality of the same compound layers can be laminated.
The present invention features a hard coating having a thickness of 0.1 to 10 xcexcm. If less than 0.1 xcexcm, the coating cannot increase the wear resistance sufficiently. If more than 10 xcexcm, the coating tends to be damaged by spalling or chipping.
It is desirable that a hard coating have a center-line mean roughness of at most 0.1 xcexcm at its surface. This specification can reduce the roughness of the machined surface of a workpiece, improving the machining precision. The surface roughness of a hard coating can be reduced by polishing the surface of the base material before coating or by polishing the surface of the hard coating after coating.
It is desirable that a hard coating be formed on at least the face of a cutting tool. In particular, the coating is the most effective when it is formed in the area from the face to the flank, because the area plays a principal role in the cutting work. However, the coating only on the face also can significantly suppress in particular, the development of crater wear.
A hard coating can be formed by the well-known PVD method. In particular, the ion-plating method and the sputtering method can form a coating having high quality bonding with the base material. In this case, the elastoplastic deformation of the hard coating is restricted at the interface with the base material while the tool is cutting. As a result, the hard coating can have further increased hardness and a high bonding strength that precludes spalling.
It is desirable that an intermediate layer comprising a compound consisting mainly of boron and at least one metal element selected from the IV a-group elements be provided between the hard coating and the base material. The presence of the intermediate layer can prolong the tool life because it intensifies the bonding strength between the cBN sintered body and the hard coating. More specifically, it is desirable that the intermediate layer comprise a boride of a IV a-group element, a boronitride of a IV a-group element, or a mixed composition position of a boride of a IV a-group element and a nitride of a IV a-group element. Of these groups, it is particularly desirable to use a boride of a IV a-group element, preferably TiB2.
It is desirable that an intermediate layer have a thickness of 0.05 to 3 xcexcm. A thickness of less than 0.05 xcexcm or more than 3 xcexcm, does not improve the bonding quality.
As with the hard coating, an intermediate layer can be formed by the PVD method, such as the ion-plating method or the sputtering method. An intermediate layer may be formed solely by the PVD method; it may also be formed by the following method: First, a metal layer of a IV a-group element is formed on the base material by the PVD method. Second, the metal layer is heat-treated to react with the cBN in the base material. This reaction forms a boride of a IV a-group element, a boronitride of a IV a-group element, or a mixed composition of a boride of a IV a-group element and a nitride of a IV a-group element.
The present invention is explained by the following examples.